A question about f-stop...

[bnnrcn]

In one of CIC's tutorials it says that ' An f-stop is defined as the ratio of the focal length to aperture diameter'.If this is the situation,how has my tamron 17-50 mm lens got a minimum f-2.8 point for all focal lengths between 17mm and 50mm(I used to have a Canon 18-55mm kit lens and the minimum f point used to change when I changed the focal length).I'm sure there is an explanation about it but I don't know and I need your help:-)

[Manfred M]

Binnur - what this means is that the aperture (iris) also has to be variable as the focal length changes to keep that constant maximum f-stop. Don't look at the maximum diameter of the front lens element, but rather the camera iris diameter when you do this calculation.

[benm]

f/stop is the ratio of the focal length to the diameter of the entrance pupil. The entrance pupil is the optical image of the aperture diameter and is not necessarily the same as the actual aperture diameter (which is what you would see if there was no lens in the way). And that's why when you set the lens to f/2.8 and 50 mm you can't get f/0.95 at 17 mm.

[bnnrcn]

So what I have understood from your answer is that,I can get f 2.8 for differrent focal lengths , the iris diameter is not same for each focal length,but it is still the widest for the relevant focal length (if 2.8 is the widest aperture for that lens ).Am I right?

Binnur - what this means is that the aperture (iris) also has to be variable as the focal length changes to keep that constant maximum f-stop. Don't look at the maximum diameter of the front lens element, but rather the camera iris diameter when you do this calculation.

[Glenn NK]

Yes

[bnnrcn]

Thank you everybody,you have helped me to clear my mind again.I'm happy to be with you in CIC:-)

[Manfred M]

Binnur - the f-stop is determined by a simple calculation:

f-stop = focal length / aperture diameter

If the f-stop is constant, and the focal length varies, the only way one can keep a constant f-stop is to make sure that the aperture changes with focal length changes.

[bnnrcn]

Thanks,this is what I have understood already from your first answer.Now another question comes to my mind,with my 18-55 mm kit lens,as far as I remember f stop used to change from 3,5 to 5,6 when I changed the focal length.With my tamron 17-50mm f stop stays at 2.8 but aperture diameter changes when I change the focal length..When I compare both lenses with the formula above there is no big difference in the aperture diameter if short focal lengths such as 17-18mm are used,but there is a big difference in the aperture diameter when we come to 50-55 mm.So a constant f-stop like f 2.8 is more important for bigger focal lengths.Am I right?

Binnur - the f-stop is determined by a simple calculation:

f-stop = focal length / aperture diameter

If the f-stop is constant, and the focal length varies, the only way one can keep a constant f-stop is to make sure that the aperture changes with focal length changes.

[bnnrcn]

When I changed my kit lens with Tamron 17-50 mm,it was mainly for getting a better quality of lens.I knew that I have also gained the f-stop advantage but I didn't know the f-stop advantage was so big before I started this thread and understood the whole subject thoroughly.Thank you CIC.

[Colin Southern]

Just look at the front of the lens whilst changing focal length - you'll see a hole changing size.

[Manfred M]

Thanks,this is what I have understood already from your first answer.Now another question comes to my mind,with my 18-55 mm kit lens,as far as I remember f stop used to change from 3,5 to 5,6 when I changed the focal length.With my tamron 17-50mm f stop stays at 2.8 but aperture diameter changes when I change the focal length..When I compare both lenses with the formula above there is no big difference in the aperture diameter if short focal lengths such as 17-18mm are used,but there is a big difference in the aperture diameter when we come to 50-55 mm.So a constant f-stop like f 2.8 is more important for bigger focal lengths.Am I right?

I wouldn't quite agree with what you are saying, but understand why you might draw that conclusion. If you look at a fast lens purely as a way of enabling shallow depth of field or a fast shutter speed to prevent motion blur, then that would be one possible conclusion.

On the other hand, a fast lens will let you shoot at a decent shutter speed at very low ISO values, which maximizes colour range and dynamic range; this applies as much to a wide angle lens as with a longer one. Same comment goes for shooting in low light conditions; faster glass tend to mean you can shoot at faster shutter speeds in low light.

I was looking at some low-light images I shot a few weeks back using my f/2.8 14-24mm lens; the exposures were running at up to 15 seconds. One stop slower would have been 30 seconds; the limit of what my camera can handle without an external timer and some more advanced techniques of establishing the proper exposure. Fast glass is nice, even with an ultra-wide angle lens.

[bnnrcn]

Thank you very much Manfred, I bought a Tokina 11-16mm f 2.8 in January because although I'm new in photography I belive that I need proper equipment for proper shooting which I hope I will manage one day..Before I bought it I started a thread which was 'Sigma 10-20mm f 4-5,6 versus Tokina 11-16mm f 2.8' and then I made a decision.After your explanations now I can understand more clearly that I didn't make a mistake at all by choosing f 2.8 lens.

I wouldn't quite agree with what you are saying, but understand why you might draw that conclusion. If you look at a fast lens purely as a way of enabling shallow depth of field or a fast shutter speed to prevent motion blur, then that would be one possible conclusion.

On the other hand, a fast lens will let you shoot at a decent shutter speed at very low ISO values, which maximizes colour range and dynamic range; this applies as much to a wide angle lens as with a longer one. Same comment goes for shooting in low light conditions; faster glass tend to mean you can shoot at faster shutter speeds in low light.

I was looking at some low-light images I shot a few weeks back using my f/2.8 14-24mm lens; the exposures were running at up to 15 seconds. One stop slower would have been 30 seconds; the limit of what my camera can handle without an external timer and some more advanced techniques of establishing the proper exposure. Fast glass is nice, even with an ultra-wide angle lens.

[bnnrcn]

Thank you very much Manfred, I bought a Tokina 11-16mm f 2.8 in January because although I'm new in photography I belive that I need proper equipment for proper shooting which I hope I will manage one day..Before I bought it I started a thread which was 'Sigma 10-20mm f 4-5,6 versus Tokina 11-16mm f 2.8' and then I made a decision.After your explanations now I can understand more clearly that I didn't make a mistake at all by choosing f 2.8 lens.

I wouldn't quite agree with what you are saying, but understand why you might draw that conclusion. If you look at a fast lens purely as a way of enabling shallow depth of field or a fast shutter speed to prevent motion blur, then that would be one possible conclusion.

On the other hand, a fast lens will let you shoot at a decent shutter speed at very low ISO values, which maximizes colour range and dynamic range; this applies as much to a wide angle lens as with a longer one. Same comment goes for shooting in low light conditions; faster glass tend to mean you can shoot at faster shutter speeds in low light.

I was looking at some low-light images I shot a few weeks back using my f/2.8 14-24mm lens; the exposures were running at up to 15 seconds. One stop slower would have been 30 seconds; the limit of what my camera can handle without an external timer and some more advanced techniques of establishing the proper exposure. Fast glass is nice, even with an ultra-wide angle lens.

[Manfred M]

Binnur - in my experience (and this goes for all lenses, not just extreme wide-angle ones); fast glass has two significant downsides.

1. These lenses tend to be more expensive that ones with smaller maximum apertures, so affordability is part of the issue; and

2. They tend to be heavier than slower lenses. They have larger and more optical components (which is why they tend to be more expensive) in order to correct for having to bend light at larger angles (due to the greater light gathering capabilities of the lens).

[bnnrcn]

You are right Manfred,I spent my money on lenses instead of a led TV but I'm happier with my lenses:-)Thanks again for the help.

Binnur - in my experience (and this goes for all lenses, not just extreme wide-angle ones); fast glass has two significant downsides.

1. These lenses tend to be more expensive that ones with smaller maximum apertures, so affordability is part of the issue; and

2. They tend to be heavier than slower lenses. They have larger and more optical components (which is why they tend to be more expensive) in order to correct for having to bend light at larger angles (due to the greater light gathering capabilities of the lens).

[rpcrowe]

I wouldn't quite agree with what you are saying, but understand why you might draw that conclusion. If you look at a fast lens purely as a way of enabling shallow depth of field or a fast shutter speed to prevent motion blur, then that would be one possible conclusion.

On the other hand, a fast lens will let you shoot at a decent shutter speed at very low ISO values, which maximizes colour range and dynamic range; this applies as much to a wide angle lens as with a longer one. Same comment goes for shooting in low light conditions; faster glass tend to mean you can shoot at faster shutter speeds in low light.

I was looking at some low-light images I shot a few weeks back using my f/2.8 14-24mm lens; the exposures were running at up to 15 seconds. One stop slower would have been 30 seconds; the limit of what my camera can handle without an external timer and some more advanced techniques of establishing the proper exposure. Fast glass is nice, even with an ultra-wide angle lens.

Piggy backing on Manfred's explanation: A faster constant aperture lens is also very helpful for creative lighting when shooting flash, especially when attempting the balance the flash with ambient light. This is both true indoors and outside...